1. Field of the Invention
The present invention relates to an ignition control apparatus that controls ignition of an internal combustion engine mainly utilized in a vehicle.
2. Description of the Related Art
In recent years, the issues such as environment preservation and fuel depletion. have been raised; measures for these issues are urgently required also in the automobile industry. The measures include, as an example, ultra-lean-combustion (sometimes referred to as stratified-lean-combustion) operation of an internal combustion engine that utilizes a stratified air-fuel mixture. However, in the stratified-lean-combustion operation of an internal combustion engine, the distribution of inflammable fuel-air mixtures may vary in the combustion chamber of the internal combustion engine; therefore, there exists a problem that the ignition plug is liable to smolder. In particular, in spray-guide-type stratified-lean-combustion operation where an unburned fuel is directly sprayed toward an ignition plug, smolder of the ignition plug occurs conspicuously.
When an ignition plug smolders, the ignition energy leaks to the ground level (referred to as a GND level, hereinafter) through conductive carbon or iron oxide that form the smolder, and hence the gap between a center electrode, which is a first electrode of the ignition plug, and a second electrode of the GND level is not led to a breakdown (sometimes referred to as a flashover, hereinafter); therefore, no spark discharge occurs, or it takes a superfluous time by the time the gap is led to a flashover where a spark discharge occurs. As a result, there is posed a defect that, for example, the output decreases because the actual ignition timing is delayed. Moreover, it takes a superfluous time by the time the gap is led to a flashover where a spark discharge occurs; thus, even should the gap between the electrodes of the ignition plug is led to a flashover, the spark discharge can be maintained just for a short time. As a result, there has been a problem that because ignition of the inflammable fuel-air mixture is not stabilized, the ignition performance or the combustion performance is deteriorated.
Furthermore, because in recent years, there has been a tendency that an ignition plug becomes slimmer or becomes to have a longer reach, the to-the-ground static capacity of the ignition plug is likely to increase; therefore, along with the effect of an increase, in the required voltage for the ignition plug, that is caused by an increase in the compression ratio of an internal combustion engine, the creation of an energy leak path due to the smolder of the ignition plug has been more and more affecting the ignition performance of the internal combustion engine.
Accordingly, in order to solve the foregoing problems caused by the smolder of an ignition plug, there has been proposed an ignition apparatus disclosed in Patent Document 1. In the conventional ignition apparatus disclosed in Patent Document 1, even at the timing when no combustion of an inflammable fuel-air mixture is carried out, the ignition plug performs a spark discharge so that a smolder of the ignition plug is prevented from developing and the smolder is burned out.